Recently, in order to address global warming, power generation by using natural energy (what is called renewable energy) such as solar photovoltaic power generation and wind power generation has been actively performed throughout the world. Output of this power generation is significantly affected by natural conditions such as the weather. Accordingly, it is predicted that an increase in the ratio of power derived from natural energy to the total output of power generation will cause problems during operation of power systems such as difficulty in maintaining frequencies and voltages. In order to address such problems, large-capacity storage batteries may be installed to achieve, for example, smoothing of output variations and load leveling.
Among large-capacity storage batteries, there is a redox flow battery. The redox flow battery is a secondary battery that includes a battery cell having a positive electrode, a negative electrode, and a membrane interposed therebetween and is configured to be charged and discharged while a positive electrode electrolyte and a negative electrode electrolyte are supplied to the battery cell. In general, such redox-flow-battery electrolytes used for redox flow batteries employ, as an active material, a metal element that undergoes a change in valence by oxidation-reduction. For example, there are an iron (Fe2+/Fe3+)-chromium (Cr3+/Cr2+)-based redox flow battery employing iron (Fe) ions as a positive electrode active material and chromium (Cr) ions as a negative electrode active material; and a vanadium (V2+/V3+−V4+/V5+)-based redox flow battery employing vanadium (V) ions as active materials for the two electrodes.